In recent years, concomitant with reduction in size of electronic apparatuses, higher integration of semiconductor packages has been increasingly required. Even in a mounting technique in which a semiconductor package is mounted on and fixed to a substrate so as to obtain electrical conduction, higher integration and higher density have been required.
Accordingly, a bonding method by a so-called BGA (Ball Grid Array) in which solder balls each used as an electrode are arranged in a lattice over an entire rear surface of a semiconductor package has drawn attention and has been practically used. In this BGA type semiconductor package, since electrodes are disposed on the entire rear surface as described above, the number of electrodes per unit area of the semiconductor package can be increased, and as a result, a significant effect of realizing high density mounting and reduction in mounting area can be obtained.
However, when the BGA method is used, as the pitches between solder balls are decreased, a so-called bridge is generated during re-flow, and as a result, short-circuiting between electrodes is liable to occur. This short-circuiting may be said as an inevitable phenomenon of micro soldering in which after a solder alloy is once melted and liquidized by heating, the alloy is solidified by cooling for bonding.
In addition, when one electronic component device is formed by bonding a plurality of necessary electronic elements together, so-called step soldering is performed in which solder materials having sequentially lower melting points are consecutively used as bonding of an electronic element is consecutively performed; however, when this step soldering is performed, solder bonding at a first step must be performed using a high temperature solder. As a practical material for the high temperature solder as described above, Pb-5Sn based solder may be mentioned by way of example; however, regulations against the use of Pb have become stricter concomitant with recent requirements for environmental conservation, and development of an alternative material has been strongly requested.
In order to solve the problems described above, an electrode-provided base substance and an electrode-bonding method thereof have been proposed. The electrode-provided base substance is a base substance including many electrodes, and these electrodes are to be bonded to electrodes of another base substance by a bonding material including composite-type metal nanoparticles as a primary material which are formed by boding and covering peripheries of metal nuclei having an average particle diameter of approximately 100 nm using an organic material (see Patent Documents 1 and 2).
The composition of a metal nanoparticle paste used as the bonding material described above is schematically shown in FIG. 12. A metal nanoparticle paste 300 is formed such that after surfaces of metal nanoparticles 301 composed, for example, of Au, Ag, or Cu having an average particle diameter of 1 to 100 nm are covered with a dispersing agent 302 (such as an amine, an alcohol, or a thiol-based material) which can coordinate with a metal element included in the metal fine particles, the metal nanoparticles 301 are stably dispersed in an organic solvent 303 (toluene, xylene, terpineol, mineral spirit, decanol, tetradecan, or the like) which is a dispersion medium. In addition, additives, such as a binder component and a reducing agent, may also be used (see Patent Document 3).
When a metal nanoparticle paste composed of a non-Pb material as described above is used, a bonding portion may be formed by firing metal fine particles at a low temperature unlike the case of a conventional solder material in which bonding is performed by solidification following melting; hence, environmental burden can be reduced, and in addition, an electronic element device having high bonding reliability can be obtained.
As examples of the electrode-provided base substance and the electrode-bonding method thereof disclosed in the Patent Documents 1 and 2, FIG. 13 shows a cross-sectional view of one example of an electronic element device obtained by flip-chip connection using the metal nanoparticle paste disclosed in the Patent Document 3, and FIG. 14 shows one example of the manufacturing method thereof.
The manufacturing process will be described with reference to FIGS. 14(a) to 14(c). First, as shown in FIG. 14(a), after a substrate 201 on which substrate electrodes 202 provided to obtain electrical conduction to an electronic element 205, a resist film 203, and dam portions 203′ are formed at an upper surface side is prepared, a metal nanoparticle paste 204 is applied onto the substrate electrodes 202.
Next, as shown in FIG. 14(b), after the electronic element 205 on which electronic element electrodes 206 provided to obtain electrical conduction to the substrate 201, a passivation film 207, and dam portions 207′ are formed at an upper surface side is prepared, the electronic element 205 is placed upside down, and the substrate 201 and the electronic element 205 are aligned so that the substrate electrodes 202 face the respective electronic element electrodes 206.
Subsequently, as shown in FIG. 14(c), after the substrate 201 and the electronic element 205 are overlapped with each other, the metal nanoparticle paste 204 is sintered by heating to form metal nanoparticle paste sintered bodies 208 for boding the substrate 201 and the electronic element 205, so that an electronic element device 200 as shown in FIG. 13 is obtained.
In addition, a technique has also been proposed in which after plating is performed on surfaces of electrodes provided on a substrate in order to improve bonding properties to metal particles, porous metal bumps used for fine pattern connection are formed by sintering the metal fine particles (see Patent Document 4).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-326416    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-128357    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-299833    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-174055
As the electronic element electrode 206 on the electronic element 205 shown in FIGS. 13 and 14 which illustrate the example of the electrode-provided base substance and the electrode-bonding method thereof disclosed in the Patent Documents 1 and 2, in consideration of electrical conductivity and workability (dimensional accuracy, working speed, and the like), in general, Al or an Al alloy has been used in many cases. However, the surface of the Al electrode is generally covered with an Al oxide film 209 as shown in FIG. 14(b).
Hence, when the bonding as described above is performed without performing any pre-treatment, as shown in FIG. 14(c) and FIG. 15 which is an enlarged view thereof, the Al oxide film 209 is actually present between the substrate electrode 202 and the electronic element electrode 206. In the case as described above, the metal nanoparticle sintered body 208 and the Al oxide film 209 are not sufficiently bonded to each other, and as a result, a problem in that the bonding reliability and the electrical conductivity are degraded may arise.
In addition, according to the method disclosed in the Patent Document 4, when Al is used as a material for the electrodes provided on the substrate, since the Al electrode is also placed in the state in which the surface thereof is generally covered with an Al oxide film as described above, first, a step of removing the Al oxide film is additionally required, and plating must be performed thereafter. Hence, the process becomes complicated, and as a result, a problem in that manufacturing cost of the electronic element device is increased may arise.